Band minority electronic carriers

When illuminated with energy equal to or above the band gap hv > Eg) at these operating potentials, minority hole carriers in n-type electrodes drive the OER at the electrode-electrolyte interface while minority electron carriers in p-type electrodes drive the HER at this interface. The potential at which this phenomenon begins to occur is the photocurrent onset potential (Eonset). which is offset relative to the flat-band potential (Efb) by the required kinetic overpotentials for the reaction of interest. The difference between the photocurrent onset potential (Eonset) and the reversible redox potential of interest (E°) is the onset voltage (Eonset)- A band diagram of a n-type photoanode and its hypothetical j-V response is shown in Fig. 6.7. 

Global AMI.5 sun illumination of intensity 100 mW/cm ). The DOS (or defect) is found to be low with a dangling bond (DB) density, as measured by electron spin resonance (esr) of - 10 cm . The inherent disorder possessed by these materials manifests itself as band tails which emanate from the conduction and valence bands and are characterized by exponential tails with an energy of 25 and 45 meV, respectively the broader tail from the valence band provides for dispersive transport (shallow defect controlled) for holes with alow drift mobiUty of 10 cm /(s-V), whereas electrons exhibit nondispersive transport behavior with a higher mobiUty of - 1 cm /(s-V). Hence the material exhibits poor minority (hole) carrier transport with a diffusion length <0.5 //m, which puts a design limitation on electronic devices such as solar cells. 

N. K. Dutta, Radiative Transitions in GaAs and Other III-V Compounds R. K. Ahrenkiel, Minority-Carrier Lifetime in III-V Semiconductors T. Furuta, High Field Minority Electron Transport in p-GaAs M. S. Lundstrom, Minority-Carrier Transport in III-V Semiconductors R A. Abram, Elfects of Heavy Doping and High Excitation on the Band Structure of GaAs D. Yevick and W. Bardyszewski, An Introduction to Non-Equilibrium Many-Body Analyses of Optical Processes in III-V Semiconductors... 

Owing to its extraordinary chemical stability, diamond is a prospective electrode material for use in theoretical and applied electrochemistry. In this work studies performed during the last decade on boron-doped diamond electrochemistry are reviewed. Depending on the doping level, diamond exhibits properties either of a superwide-gap semiconductor or a semimetal. In the first case, electrochemical, photoelectrochemical and impedance-spectroscopy studies make the determination of properties of the semiconductor diamond possible. Among them are the resistivity, the acceptor concentration, the minority carrier diffusion length, the flat-band potential, electron phototransition energies, etc. In the second case, the metal-like diamond appears to be a corrosion-stable electrode that is efficient in the electrosyntheses (e.g., in the electroreduction of hard to reduce compounds) and electroanalysis. Kinetic characteristics of many outer-sphere... 

In the case of a cathodic reaction via the valence band, minority carriers are injected into an n-type electrode in the dark. The injected holes diffuse into the bulk of the semiconductor until they recombine with the electrons (Fig. 32), and the current is then determined by the difference of the two quasi-Fermi levels Ep, and Efr.p, i.e. 

Semiconductor electrodes whose band gap is relatively narrow receive photon energy and produce photoexcited electron-hole pairs in the space charge layer. The photoexcited electron-hole pair formation significantly increases the concentration of minority charge carriers (holes in the n-type), but influences little the concentration of majority carriers (electrons in the n-type). The photoexcited electrons and holes set their energy levels not at the electrode Fermi level, ef, but at what we call the quasi-Fermi levels, n p and p p, respectively. The quasi-Fermi level for majority carriers is close to the electrode Fermi level, F, but the quasi-Fermi level for minority carriers is far away from the electrode Fermi level. 

Possibilities for indirect activation of surface states by irradiation can also be distinguished. Firstly, minority charge carriers created within the bulk valence/conduction band by irradiation may become localised at surface states through electron tunnelling between them and the bulk, as in eqn. (10) or eqn. (11), provided that there is a good match between the energy levels involved in the bulk and at the surface, viz. 

On the absorption of a photon by a n-type semiconductor, one electron is excited from the valence band and promoted to the conduction band [see Figure 4.9(b)] to leave a net positive charge - a hole (h" ) - in the valence band. The electron in the conduction band is forced to the back contact and is transferred to the counter electrode. At the electrode I electrolyte interface of this latter electrode, the electrons react with protons to produce hydrogen, while the simultaneously created holes in the valence band of the semiconductor (the minority carriers) generate oxygen by oxidizing water. Thus, the electrode half-reactions in the PEC cell are as follows ... 

Introduction The dynamics of electron-hole recombination at the semiconductor surface has been extensively studied both at illuminated and at dark s/e interfaces [45-53]. For recombination, minority charge carriers should be present at the interface. For n-type semiconductors, holes may be supplied to the surface by illumination under depletion conditions using supra-band gap light (see Sect. 2.1.2.2). Alternatively, holes may be injected into the valence band by a strong oxidizing agent in the electrolyte solution. 

The photoproduction and subsequent separation of electron-hole pairs in the depletion layer cause the Fermi level in the semiconductor to return toward its original position before the semiconductor-electrolyte junction was established, i.e., under illumination the semiconductor potential is driven toward its flat-band potential. Under open circuit conditions between an illuminated semiconductor electrode and a metal counter electrode, the photovoltage produced between the electrodes is equal to the difference between the Fermi level in the semiconductor and the redox potential of the electrolyte. Under close circuit conditions, the Fermi level in the system is equalized and no photovoltage exists between the two electrodes. However, a net charge flow does exist. Photogenerated minority carriers in the semiconductor are swept to the surface where they are subsequently injected into the electrolyte to drive a redox reaction. For n-type semiconductors, minority holes are injected to produce an anodic oxidation reaction, while for p-type semiconductors, minority electrons are injected to produce a cathodic reduction reaction. The photo-generated majority carriers in both cases are swept toward the semiconductor bulk, where they subsequently leave the semiconductor via an ohmic contact, traverse an external circuit to the counter electrode, and are then injected at the counter electrode to drive a redox reaction inverse to that occurring at the semiconductor electrode. 

A material of this type is said to be an n-type extrinsic semiconductor. The electrons are majority carriers by virtue of their density or concentration holes, on the other hand, are the minority charge carriers. For n-type semiconductors, the Fermi level is shifted upward in the band gap, to within the vicinity of the donor state its exact position is a function of both temperature and donor concentration. 

The direct and indirect behaviors mentioned above are sufficiently important that they deserve special mention. The critical aspects of the energy band structures of these two types of semiconductor are shown schematically in Figure 2.8. The minimum energy of the conduction band in indirect materials is at a different momentum than that of the maximum energy of the valence band. Electrons in the conduction band rapidly relax to the minimum band energy. Holes equally rapidly move to the maximum energy of the valence band. Therefore, electrons and holes do not normally have the same momentum in an indirect semiconductor while in a direct-gap material these momenta are equal. This has consequences for the minority carrier lifetimes and optical properties of semiconductors. 

Figure C2.16.7. A schematic energy band diagram of a p-n junction witliout external bias (a) and under forward bias (b). Electrons and holes are indicated witli - and + signs, respectively. It should be remembered tliat tlie energy of electrons increases by moving up, holes by moving down. Electrons injected into tlie p side of tlie junction become minority carriers. Approximate positions of donor and acceptor levels and tlie Feniii level, are indicated.

The impurity atoms used to form the p—n junction form well-defined energy levels within the band gap. These levels are shallow in the sense that the donor levels He close to the conduction band (Fig. lb) and the acceptor levels are close to the valence band (Fig. Ic). The thermal energy at room temperature is large enough for most of the dopant atoms contributing to the impurity levels to become ionized. Thus, in the -type region, some electrons in the valence band have sufficient thermal energy to be excited into the acceptor level and leave mobile holes in the valence band. Similar excitation occurs for electrons from the donor to conduction bands of the n-ty e material. The electrons in the conduction band of the n-ty e semiconductor and the holes in the valence band of the -type semiconductor are called majority carriers. Likewise, holes in the -type, and electrons in the -type semiconductor are called minority carriers. 

A more effective carrier confinement is offered by a double heterostmcture in which a thin layer of a low band gap material (the active layer) is sandwiched between larger band gap layers. The physical junction between two materials of different band gaps, and chemical compositions, is called a heterointerface. A schematic representation of the band diagram of such a stmcture is shown in Figure 4. Electrons injected under forward bias across the p—N junction into the lower band gap material encounter a potential barrier, AE at thep—P junction which inhibits their motion away from the junction. The holes see a potential barrier of AE at the N—p heterointerface which prevents their injection into the N region. The result is that the injected minority... 

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